1. Field of the Invention
The present invention relates generally to bearings and, more particularly in order of increasing specificity, to hydrodynamic bearings, compliant bearings, foil bearings, and foil thrust bearings.
2. Related Art
The load capacity of a foil thrust beating depends on the compliance of the bearing with pressure exerted by a fluid film developed between the bearing and the associated runner, or rotatable member of the device incorporating the bearing. The pressure profile for a thrust beating varies, and in order to accommodate the optimum pressure profile and attendant fluid film thickness associated with maximum lead capacity, the thrust bearing should be designed to provide stiffness which varies in a manner similar to the pressure profile. Prior foil thrust bearings have been known to exhibit limited lead capacity resulting from the incorporation of springs designed with limited accommodation for the variance in fluid pressure profile and the resultant effect on lead capacity of the bearing. A typical spring utilized in such bearings is illustrated in FIGS. 2-6 of U.S. Pat. No. 4,668,106 issued to Gu. While such spring designs provide varying stiffness in radial directions, they provide limited lead capacity because of excess pad deflection over the spring support points. The excessive pad deflection loads to a divergent fluid film at the trailing edge of the pad and prevents the bearing from developing an optimum pressure profile.
More recent foil thrust bearings, such as those illustrated in U.S. Pat. No. 5,318,366 issued to Nadjafi and U.S. Pat. No. 5,248,205 issued to Gu, et al. incorporate springs which are configured to provide a variation in bearing stiffness in both the radial and circumferential directions. While these thrust bearings solve the aforementioned problems with earlier bearings, they are not suited for use in bi-directional devices, i.e. those in which the runner disposed adjacent the bearing may rotate in either direction. This occurs since, in one direction of rotation, there is a mismatch between the boating stiffness profile and the fluid film pressure profile, causing a deterioration in the conformity of the bearing shape to the rotatable member, or runner.
Accordingly, prior to the present invention, engineer continue to search for improved foil thrust boating designs.